IMPALA-6027: Retry downloading toolchain components.
IMPALA-6027: Retry downloading toolchain components.

We've seen intermittent 500 errors when downloading the toolchain from
S3 over the HTTPS URLs. As a first stab, this commit retries 3 times,
with some jitter.

I also changed the threadpool introduced previously to have a limit
of 4 threads, because that's sufficient to get the speed improvement.
The 500 errors have been observed both before and after the threadpool
change.

For testing, I ran the straight-forward case directly. I introduced
a broken version string to observe that retries would happen on
any error from wget.

Change-Id: I7669c7d41240aa0eb43c30d5bf2bd5c01b66180b
Reviewed-on: http://gerrit.cloudera.org:8080/8258
Reviewed-by: Thomas Tauber-Marshall <55b5d0aded728f7a3075d2dcc6abf84f96c36406@cloudera.com>
Reviewed-by: Michael Brown <97e4d5d0e400e5ad37382425ea95bde8b3933403@cloudera.com>
Tested-by: Impala Public Jenkins

cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala